In wireless radio communication systems, such as cellular telephone networks, it is generally recognized that there is a great need and utility for accurately determining in real time the location of mobile radio transmitters, such as cellular phone handsets. For example, U.S. Pat. No. 5,512,908 to Herrick mentions the application of cellular location information to 911 dispatching, tracking unauthorized cell phone usage, and tracking or locating commercial and/or government vehicles. U.S. Pat. No. 5,327,144 to Stilp et al. also mentions various applications of mobile location information, such as locating lost or stolen vehicles, assisting lost motorists, and dispatching emergency vehicles.
All of the above location-based services, however, fundamentally depend on the ability to obtain consistent and accurate location information in a wide range of environments. Although conventional techniques for location finding can provide location information in certain limited situations, they fail to provide accurate location information in many environments, particularly in urban environments where signal multipath is often severe. Urban environments, however, are the places where such services are often needed most.
For example, in U.S. Pat. No. 5,570,412 LeBlanc discloses a wireless location finding system using a databank of location signatures. These signatures are conventional RF signal parameters derived from signals received at a single antenna, and they have little correlation with location, with the exception of time differential data. This time data, however, is severely degraded in high multipath environments Consequently, the techniques taught by LeBlanc are of limited use in urban environments where multipath is high. In addition the time differential data can be measured only by multiple base stations. This requires expensive and precise communication and synchronization between multiple base stations, and also limits the utility of the technique to the rare situations where the transmitter is in simultaneous range of multiple base stations.
In contrast to conventional techniques for location finding, Hilsenrath et al. in U.S. patent application Ser. No. 08/780,565 (which is not admitted to be prior art by its mention in this background discussion) disclose a unique method for location finding that performs especially well in multipath environments, and requires only a single base station. While conventional wisdom views multipath signals as noise to be reduced, ignored, or eliminated, the method of Hilsenrath et al. takes advantage of multipath signals to help identify transmitter locations. Because signal signatures are naturally associated with locations by virtue of the spatial dependence of multipath, the base station can use this fact to determine a location by matching a measured signature with a particular calibrated signature in a database of signatures and corresponding locations.
Due to noise, variable transmitter movement, noise, and other error sources, it can be difficult to consistently obtain accurate and reliable signal signature measurements. There is a need, therefore, to provide techniques for improving the reliability and accuracy of signal signature measurements in these location finding systems.